Reverse osmosis membranes of polyamideurethane

ABSTRACT

The present invention is directed to an improved reverse osmosis membrane that shows surprisingly improved solute rejection and permeation properties. The membrane includes a separating layer of a polyamideurethane formed in situ by reaction of a haloformyloxy-substituted acyl haldide with a diamine-treated substrate.

FIELD OF THE INVENTION

This invention relates to composite membranes for use in reverse osmosisprocesses such as the desalination of aqueous solutions. Moreparticularly, the present invention relates to a multilayer membrane inwhich one layer is a copolymer of polyamideurethane.

BACKGROUND OF THE INVENTION

Reverse osmosis is a well known process for purification of salinewater. In this process, a pressure in excess of the osmotic pressure ofthe saline water feed solution is applied to the feed solution toseparate purified water by use of a permselective semipermeablemembrane. Purified water is thereby caused to diffuse through themembrane while salt and other impurities are retained by the membrane.

Permselective membranes include composite membranes that include aseparating layer on a supporting microporous substrate. The substrate istypically supported on a porous support to impart mechanical strength tothe membrane. Permselective membranes suitable for use in reverseosmosis are available in various forms and configurations. Flat sheet,tubular and hollow fiber membranes are well-known in the art. Thesemembranes can also vary in morphology. Homogenous and asymmetricmembranes are operable, as well as thin film composites.

Permselective membranes are available in the form of multi-layerstructures that include a membrane layer superimposed on a microporoussubstrate. Membrane layers which may be employed over the substrateinclude polyamides, polyphenylene esters, and polysulfonamides.

Polyamide discriminating layers are well-known in the art. The polyamidecan be aliphatic or aromatic and may be crosslinked. Polyamide membranesmay be made by the interfacial reaction of a cycloaliphatic diamine withisophthaloyl chloride, trimesoyl chloride or mixtures of these acidchlorides. Polyamide membranes also may be made by reaction ofm-phenylene diamine and cyclohexane-1,3,5-tricarbonyl chloride. Inaddition, polyamide membrane also may be made by reaction of aromaticpolyamines having at least two primary amines on an aromatic nucleus,and an aromatic polyfunctional acyl halides having an average of morethan two acyl halide groups on an aromatic nucleus.

These prior art membranes, although useful as reverse osmosis membraneshave, however, been prone to deficiencies such as short useful life, aswell as low flux and low salt rejection. A need therefore exists forimproved reverse osmosis membranes which show both high rates of saltrejection while providing improved rates of flux.

SUMMARY OF THE INVENTION

The present invention is directed to an improved reverses osmosismembrane that shows surprisingly improved solute rejection andpermeation properties. The membrane includes a separating layer of apolyamideurethane formed in situ by reaction of ahaloformyloxy-substituted acyl chloride with a diamine-treatedsubstrate.

In accordance with the present invention, the improved reverse osmosismembranes are formed by treating a polymeric microporous substrate witha solution of a diamine. The treated substrate then is exposed to ahaloformyloxy-substituted acyl halide in an organic solvent that isnon-reactive with the polymeric substrate to provide a membrane ofpolyamideurethane.

The resulting membrane's surprisingly improved solute rejection andpermeation properties enable the membrane to be employed in a widevariety of applications where high purity permeate is required. Examplesof these applications include, but are not limited to, desalination ofsalt water, purified water for semiconductor manufacturing, reduction ofBOD in waste water treatment, removal of dissolved salts during metalrecovery, dairy processing, fruit juice concentration, de-alcoholizationof wine, beer, and the like. In such applications, the liquid is placedunder pressure while in contact with the improved membranes of theinvention to remove impurities.

DETAILED DESCRIPTION OF THE INVENTION

Having briefly summarized the invention, the invention will now bedescribed in detail by reference to the following specification andnon-limiting examples. Unless otherwise specified, all percentages areby weight and all temperatures are in degrees centigrade.

Generally, the manufacture of the improved reverse osmosis membranes ofthe invention is accomplished by treating a polymeric microporoussubstrate with an aqueous solution of a polyfunctional amine such asm-phenylenediamine, piperazine, xylylenediamine, and the like,preferably an aromatic diamine such as p-phenylenediamine,m-phenylenediamine, and the like, most preferably m-phenylenediamine,and further treating the substrate with a solution of ahaloformyloxy-substituted acyl halide such as5-chloroformyloxyisophthaloyl chloride, 4-chloroformyloxyisophthaloylchloride, 2-chloroformyloxyisophthaloyl chloride, bromo analogs of5-chloroformyloxyisophthaloyl chloride such as5-bromoformyloxyisophthaloyl dibromide, 5-bromoformyloxyisophthaloylchloride, preferably 5-haloformyloxyisophthaloyl dihalides, mostpreferably, 5-chloroformyloxyisophthaloyl chloride. The reaction of thehaloformyloxy-substituted acyl halide with the polyfunctional amineprovides a novel composition of a polyamideurethane that shows bothsurprisingly improved solute rejection and improved solvent flux. Thegeneral formula of the polyamideurethane is: ##STR1## where X=trivalentorganic group such as tri substituted cyclo hexane, tri substitutedbenzene, tri substituted naphthalene, tri substituted cyclo pentane, trisubstituted cyclo heptane and the like, and

Y=divalent organic group such as m-phenylene diamine, p-phenylenediamine, piperazine and the like.

Generally, the haloformyloxy-substituted isophthaloyl chlorides may beprepared by reacting an hydroxy-substituted isophthalic acid, or saltsof hydroxy-substituted isophthalic acid, catalyst, phosgene, and asolvent under autogeneous pressure at elevated temperature. Preferably,the 5-chloroformyloxyisophthaloyl chloride (CFIC) that is mostpreferably reacted with the diamine-treated microporous substrate isprepared by heating a mixture of 25 g of 5-hydroxyisophthalic acid, 0.3g of imidazole, 70 g of phosgene, and 100 ml of chlorobenzene solvent ina pressure vessel at 160° C. for 18 hours under autogeneous pressure.Removal of the solvent, followed by distillation of the product at143°-151° C. and 1 mm Hg yields 12.6 g of (CFIC) (white solid, mp:55.5°-56.5° C.).

CFIC also may be produced by using alternatives to the preferredreactants mentioned above. For example, salts of 5-hydroxyisophthalicacid such as disodium 5-hydroxyisophthalate or trisodium5-hydroxyisophthalate may be substituted for 5-hydroxyisophthalic acid.Similarly, imidazole may be replaced with other heteroatom-containingcompounds capable of complexing phosgene. Examples of such catalystsinclude, but are not limited to, pyridine, N,N-dimethylformamide (DMF),N,N-dimethylacetamide (DMAc), and the like. Likewise, solvents such asdioxane and methylene chloride may be employed, so long as the solventis reasonably unreactive with the reactants and products.

CFIC is most preferred for reacting with the diamine-treated substrateto effect interfacial polymerization of polyamideurethane. However,analogs such as 5-bromoformyloxyisophthaloyl bromide, may be substitutedfor CFIC. Positional isomers of CFIC such as4-chloroformyloxyisophthaloyl chloride may be substituted for CFIC.Aliphatic analogs, such as 5-chloroformyloxycyclohexane-1,3-dicarbonylchloride may be employed as well. The haloformyloxy-substituted acylhalide also may be employed in combination with a diacyl halide toeffect polymerization with a diamine to polyamideurethane; isophthaloylchloride is an example of such a diacyl halide.

Generally, the membranes of the present invention can be manufactured byfirst casting a suitable substrate layer for the membrane onto a supportmembrane. Suitable substrate layers have been described extensively inthe art. Illustrative substrate materials include organic polymericmaterials such as polysulfone, polyethersulfone, chlorinated polyvinylchloride, styrene/acrylonitrile copolymer, polybutylene terephthalate,cellulose esters and other polymers which can be prepared with a highdegree of porosity and controlled pore size distribution. Thesematerials are generally cast onto a support material of non-woven fabricor woven cloth, generally of polyester or polypropylene. Porous organicand inorganic materials also may be employed as the support material.Examples of possible support materials include, but are not limited to,nylon, cellulose, porous glass, ceramics, sintered metals, and the like.These support materials may be in the form of flat sheets, hollow tubes,hollow fibers, and the like to provide, for example, membranes in theform of fibers.

Preparation of microporous polymeric substrates is well known in theart. Preparation of microporous polysulfone that is the preferredsubstrate typically includes casting a solution of 15-20% polysulfone indimethylformamide (DMF) onto a support member, followed by immediatelyimmersing the cast material into water to produce microporouspolysulfone film. The side of the polysulfone film exposed to the airduring casting is called the "face" and contains very small pores,mostly under 200 angstroms in diameter. The "back" of the film incontact with the support member has very coarse pores.

After casting, the porous polysulfone substrate is treated with anaqueous polyfunctional amine, preferably, a polyfunctional aromaticamine. Aqueous m-phenylenediamine (MPD) is most advantageously employedto treat the substrate. However, other aromatic amines with sufficientwater solubility to effect interfacial polymerization withhaloformyloxy-substituted acyl halides also may be employed. Examples ofdiamines include but are not limited to p-phenylenediamine, piperazine,m-xylylenediamine, and the like. The amine-impregnated substrate is thenexposed to haloformyloxy-substituted acyl halide.

In the following illustrative examples, the microporous polysulfonesubstrate is exposed to an aqueous solution of m-phenylenediamine (MPD)of indicated weight/volume (w/v) percent concentration at a temperatureof 20° C. for 2 to 5 minutes. Advantageously, 0.5 to 3.0% by weight ofaqueous MPD, and most advantageously 1 to 2% by weight of aqueous MPD,is employed. After exposure, the substrate is removed from the MPDsolution, drained, and excess MPD solution removed from the substratewith a rubber roller. The MPD-treated polysulfone substrate then isexposed to a solution of a water-immiscible solvent containing ahaloformyloxy-substituted acyl halide, preferably a solution of CFIC,under conditions conducive to polymerization of the polyamideurethanemembrane. Suitable solvents for the haloformyloxy-substituted acylhalide are solvents which do not deleteriously affect the substrate.Examples of solvents include, but are not limited to C₅ -C₈ n-alkanes,C₄ -C₈ fluoroalkanes, C₅ -C₈ chlorofluoroalkanes, C₅ -C₈ cycloalkanes,C₂ -C₆ chlorofluoroalkanes, and C₄ -C₈ cyclo chlorofluoroalkanes, andFreon TF (1,1,2-trichlorotrifluoroethane). Most preferably, Freon TF isemployed as the solvent for the CFIC solution.

The concentration of CFIC in the solution that is necessary to effectinterfacial polymerization of polyamidurethane on the diamine-treatedsubstrate can vary depending on the specific solvent, substrate, and thelike, and can be determined experimentally. Generally, however, CFICconcentrations of 0.03-5%, preferably 0.05-0.20%, can be employed.

After formation of the polyamideurethane membrane layer, the resultingmembrane is removed from the CFIC solution and drip dried for 5 to 120seconds, preferably 60 to 120 seconds, most preferably for 120 seconds.The membrane then is treated to extract impurities such as residualCFIC, residual diamine, reaction by products, and the like. This isaccomplished by successively treating the membrane with water, andaqueous alkanol. Accordingly, the membrane is washed in running tapwater at 20° to 60° C. preferably 40° to 60° C., most preferably 50°-55°C., for 5 to 30 minutes, preferably 10 to 20 minutes, most preferablyten minutes, and then in an aqueous lower C₁₋₃ alkanol, such asmethanol, ethanol, isopropanol, preferably ethanol. The aqueous ethanolemployed may be 5 to 25% ethanol, preferably 10 to 15% ethanol, mostpreferably 15% ethanol, the remainder being water. The aqueous ethanolis at 20° to 60° C., preferably 40° to 60° C., most preferably 50°-60°C. The membrane is washed in aqueous alkanol for 5 to 20 minutes,preferably 10 to 20 minutes, most preferably ten minutes. The membraneis then water-rinsed to remove ethanol.

The membrane then is stored in damp until testing for permeability andflux. Alternatively the membrane may be impregnated with a wetting agentsuch as glycerine to provide for dry storage and subsequent rewetting.

The membranes of the invention may be made in a variety ofconfigurations and can be assembled in a variety of devices. Preferably,the membranes are in the form of films and fibers. For example, flatsheets of the membrane can be utilized in either plate and frame orspiral devices. Tubular and hollow fiber membranes can be assembled ingenerally parallel bundles in devices with tubesheets at opposing endsof the membranes. Radial, axial or down the bore flow feed can beutilized in hollow fiber devices.

The resulting membranes of polyamideurethane on a polymeric substratesuch as polysulfone are evaluated for salt rejection and flux bysubjecting the membranes to a feed of aqueous 0.26%-0.28% NaCl at pH 6.8and 25°-30° C. in a cross flow permeation cell. Membranes measuring 47mm diameter are placed into the cell and exposed to 0.75 l/minute of theaqueous NaCl solution. The membranes are exposed to feed pressure of 420psig for at least 14 hours, after which the feed pressure is lowered to225 psig, and the permeation properties determined. The performance ofthe membrane is characterized in terms of the percent of salt NaClrejected (R), permeability (Kw), and permeate productivity. The percentsalt rejected is defined as

    R=(1-(C.sub.p /C.sub.f))*100%

where C_(p) and C_(f) are concentrations of NaCl in the permeate andfeed, respectively. The concentration of the NaCl in the permeate andfeed can be determined conductimetrically with a Beckman G1 conductivitycell (cell constant of 1.0), and a YSI Model 34 conductivity meter.

The permeability (Kw), defined as (flux/effective pressure), where fluxis the water flow rate through the membrane and the effective pressureis equal to the feed pressure minus the opposing osmotic pressure. Fluxis expressed in terms of permeate productivity, that is, in terms of(gallons of permeate/square foot membrane area/day), (GFD) at 25° C. and225 psig. Correspondingly, permeability is expressed in terms ofmeters/second/teraPascal (m/s/TPa). The values of permeability, saltrejection and productivity of the membranes are given below. Conversion,expressed as volume of permeate per unit time divided by volume of feedper unit time is typically below 2%.

The membranes of this invention can be readily tailored specificapplications such as removal of salt from potable water, dairyprocessing, and the like by varying, for example, the concentration ofthe haloformyloxy-substituted acyl halide employed to treat the diaminetreated substrate. Accordingly, polyamideurethane layers may be formedthat are suitable for achieving salt rejections from below 90 percent tomore than 99 percent.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever. In the followingexamples, all temperatures are set forth in degrees centigrade; unlessotherwise indicated, all parts and percentages are by weight.

EXAMPLES 1-10

A microporous polysulfone substrate is prepared by casting a 16%solution of UDEL P3500 polyethersulfone from Union Carbide Corp. inN,N-dimethylformamide (DMF) containing 0.3% water onto a support ofpolyester sailcloth. The solution is cast at a knife clearance of 5.5mil. The sailcloth bearing the cast polyethersulfone solution isimmersed in a water bath within 2 seconds of casting to produce amicroporous polysulfone substrate. The substrate is water-washed toremove the N,N-dimethylformamide solvent and is stored damp until use.

The microporous polysulfone substrate is immersed in an aqueous solutionof metaphenylenediamine (MPD) of indicated concentration for 5 minutes.The substrate is removed, drained briefly and rolled with a rubberroller to remove surface droplets of excess MPD. The MPD-impregnatedsubstrate then is immersed in a solution of5-chloroformyloxyisophthaloyl chloride (CFIC) in FREON TF solvent(1,1,2-trichlorotrifluoroethane) of indicated concentrations for 20-40seconds to form a membrane of polyamideurethane.

The membrane is removed from the CFIC solution and drip dried for 2minutes. The membrane then is successively treated in hot (55° C.)running tap water for 10 minutes, and then in stirred 15% aqueousethanol (50°-60° C.) for 10 minutes. The membrane is stored in watercontaining 0.1% sodium bicarbonate until testing for permeability andflux. The performance of the membranes formed with CFIC in solvent isreported in Table 1.

                  TABLE 1                                                         ______________________________________                                                               %                Produc-                               Ex-   MPD      CFIC    NaCl             tivity                                ample Conc     Conc    Rejec- Permeability                                                                            (gfd @                                #     (%)      (%)     tion   Kw (m/s/TPa)                                                                            225 psig)                             ______________________________________                                        1     1.0      0.05    99.29  5.48      15.4                                  2     2.0      0.05    99.20  3.96      11.5                                  3     1.0      0.10    99.47  4.18      11.9                                  4     1.2      0.10    99.44  2.85      8.1                                   5     1.5      0.10    99.51  3.47      9.8                                   6     1.8      0.10    99.31  2.73      7.8                                   7     2.0      0.10    98.95  3.98      11.3                                  8     1.0      0.15    99.76  2.92      8.2                                   9     1.5      0.15    99.74  3.31      9.3                                   10    2.0      0.15    99.47  3.19      9.1                                   ______________________________________                                    

The effect of feed pH on NaCl rejection is determined for the membranesof Examples 4 and 5 by adjusting the 0.27% NaCl feed pH with HCl andNaOH. The results are given in Table 2.

                  TABLE 2                                                         ______________________________________                                                         pH 6.8  pH 3.5                                                                              pH 4.0                                                                              pH 4.9                                                                              pH 6.8                             Ex-              %       %     %     %     %                                  ample Membrane of                                                                              NaCl    NaCl  NaCl  NaCl  NaCl                               #     Example #  Rej     Rej   Rej   Rej   Rej                                ______________________________________                                        11    4          99.34   89.40 95.18 99.17                                    12    5          99.64   91.89 95.24 99.34 99.70                              ______________________________________                                    

Examples 13-16 set forth in Table 3 illustrate the performance ofmembranes produced by treating an MPD-treated support with a CFICsolution of indicated concentration that includes the indicatedconcentration of a 70:30 blend of iso- and terephthaloyl (I/T) chlorideunder the conditions of Examples 1-10.

                  TABLE 3                                                         ______________________________________                                                                    %     Perme-  Produc-                             Ex-   MPD     CFIC    I/T   NaCl  ability tivity                              ample Conc    Conc    Conc  Rejec-                                                                              Kw      (gfd @                              #     %       %       %     tion  (m/s/TPa)                                                                             225 psig)                           ______________________________________                                        13    1.0     0.05    0.10  98.53 3.5     10.0                                14    1.0     0.15    0.10  99.64 3.0     8.5                                 15    2.0     0.05    0.10  98.70 3.0     8.5                                 16    2.0     0.15    0.10  99.77 3.2     9.0                                 ______________________________________                                    

Examples 17-18 illustrate the surprising ability of the membranes ofthis invention to remove silica impurities from a feed stock. Theresults shown in Table 4 illustrate the dissolved silica rejectionachieved for the membranes of Examples 1 and 16. Silica rejection isdetermined by adding 130 ppm of sodium metasilicate nonahydrate to 0.27%NaCl aqueous feed to give 27 ppm dissolved silica as SiO₂. Silicarejection is determined at 225 psig as described above for NaClrejection. Silica concentration in the feed and permeate is determinedby Method B of ASTM D 859. The % silica rejection is given below.

                  TABLE 4                                                         ______________________________________                                        Example #                                                                              Membrane of Example                                                                            % Silica Rejection                                  ______________________________________                                        17        1               99.89                                               18       16               99.53                                               ______________________________________                                    

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

I claim:
 1. A method comprising: manufacturing a reverse osmosismembrane that shows improved solute rejection and solvent flux,by,casting a polymeric solution into a support to provide a microporouspolymeric substrate treating said substrate with a polyfunctional amineto provide an impregnated substrate, and treating said impregnatedsubstrate with a solution of a haloformyloxy-substituted acyl halide toprovide a reverse osmosis membrane with a polyamideurethane separatinglayer that shows improved solute rejection and solvent flux.
 2. Themethod of claim 1 wherein said support is selected from the group ofporous glass, sintered metal, ceramics, and organic polymers.
 3. Themethod of claim 2 wherein said organic polymers are selected from thegroup of polyolefins, polyesters, and polyamides.
 4. The method of claim1 wherein said polyfunctional amine is selected from the group ofm-phenylenediamine, p-phenylenediamine, piperazine, m-xylylenediamine,or mixtures thereof.
 5. The method of claim 4 wherein saidhaloformyloxy-substituted acyl halide is selected from the group of5-bromoformyloxyisophthaloyl dibromide, 4-chloroformyloxyisophthaloylchloride, 2-chloroformyloxyisophthaloyl chloride, and5-chloroformyloxyisophthaloyl chloride.
 6. The method of claim 5 whereinsaid haloformyloxy-substituted acyl halide is employed in combinationwith isophthaloyl chloride, terephthaloyl chloride or a blend thereof.7. The method of claim 6 wherein said haloformyloxy-substituted acylhalide is 5-chloroformyloxy-isophthaloyl chloride.
 8. The method ofclaim 5 wherein said solution of haloformyloxy-substituted acyl halideincludes a solvent selected from the group of1,1,2-trichlorotrifluoroethane, n-alkanes, and cycloalkanes.
 9. Themethod of claim 8 wherein said solvent is1,1,2-trichlorotrifluoroethane.
 10. The method of claim 4 wherein saidpolyfunctional amine is m-phenylenediamine.
 11. The method of claim 10wherein said haloformyloxy-substituted acyl halide is5-chloroformyloxyisophthaloyl chloride.
 12. The method of claim 11wherein said polymeric substrate is polysulfone.
 13. The method of claim1 wherein said polyamideurethane is of the formula: ##STR2## where X=atrivalent organic group, andY=a divalent organic group.
 14. The methodof claim 13 wherein said X is selected from the group of tri substitutedcyclo hexane, tri substituted benzene, tri substituted naphthalene, trisubstituted cyclo pentane, tri substituted cyclo heptane.
 15. The methodof claim 14 wherein said Y is selected from the group of m-phenylenediamine, p-phenylene diamine, piperazine.
 16. The method of claim 13wherein said X is tri substituted benzene and said Y is m-phenylenediamine.
 17. A reverse osmosis membrane comprising a polyamideurethaneseparating layer on a polymeric substrate having properties enablingimproved salt rejection, flux and productivity and having properties forenabling improved salt rejection, flux and productivity.
 18. The reverseosmosis membrane of claim 17 wherein said polyamideurethane is of theformula: ##STR3## where X=a trivalent organic group, andY=a divalentorganic group.
 19. The reverse osmosis membrane of claim 18 wherein saidsubstrate is polysulfone.
 20. The reverse osmosis membrane of claim 19wherein said membrane has a percent salt rejection of at least about 90percent.
 21. The reverse osmosis membrane of claim 18 wherein saidmembrane has a percent salt rejection of at least about 99 percent. 22.The membrane of claim 18 wherein said X is selected from the group oftri substituted cyclo hexane, tri substituted benzene, tri substitutednaphthalene, tri substituted cyclo pentane, tri substituted cycloheptane.
 23. The membrane of claim 22 wherein said Y is selected fromthe group of m-phenylene diamine, p-phenylene diamine, piperazine. 24.The membrane of claim 18 wherein X is tri-substituted benzene and Y ism-phenylene diamine.
 25. The reverse osmosis membrane of claim 17further comprising a polymeric substrate in contact with a supportmember selected from the group of porous glass, sintered metal,ceramics, and organic polymer.
 26. The reverse osmosis membrane of claim25 wherein said organic polymer is polyester.
 27. The reverse osmosismembrane of claim 17 wherein said membrane is in the form of a fiber.